Fat vs. Muscle: Understanding Body Composition

You are much more than just a number on the scale. While society has taught us to obsess over pounds, body weight and body mass index (BMI) do not fully define your health. To get a more complete picture, you need to evaluate your body composition.

Body composition refers to several different body measurements, including body fat percentage, muscle mass, protein rate, body cell mass, inorganic salt content and metabolic age, to name a few. These components reveal much more about your health and fitness than body weight alone. That's why it's important to have a basic understanding of body composition and how you can change it.

In the meantime, please don't forget about all your other amazing qualities—like a bright smile and contagious laugh. Your healthy body is nothing without a healthy mind and happy spirit.

What are the 4 components of body composition?

It's easy to get overwhelmed by all the different body composition markers. However, if you focus on the four main components, your health will be in good hands:

1. Fat mass

Fat mass refers to the weight of subcutaneous fat and visceral fat, combined. Subcutaneous fat lies just beneath your skin, while visceral fat is what surrounds your organs. Excessive accumulation of fat mass, especially around your organs, can impact your health. This number is expressed in pounds or kilograms of body weight but can also be recorded as a percentage of total body weight.

2. Lean (muscle) mass

When reported in pounds or kilograms, this is the total weight of skeletal muscle, smooth muscle and cardiac muscle. Lean muscle mass is responsible for all body movement, including the contractions that pump your heart. Muscle mass is more metabolically active than fat mass, meaning it burns more calories at rest and is a major contributor to your basal metabolic rate. Like fat mass, lean mass is often recorded as pounds but can also be expressed as a percentage of total body weight.

Pro tip: There's a difference between lean mass and fat-free mass. Fat-free mass includes the total weight of lean mass, bone mass and water.

3. Bone mass

Also referred to as skeletal mass, this is the total weight of your bones. Bone mass, along with bone mineral content, are important biomarkers of overall bone health. Typically, a higher bone mass indicates stronger, denser bones. Bone mass is always recorded in pounds.

4. Body water

Water is essential for almost all bodily functions, including regulating body temperature, transporting nutrients and removing waste. Tracking your total body water is a great way to monitor your hydration status and overall health. This number is always recorded as a percentage of total body weight.

How can you measure your body composition?

Body composition analysis can be done using several different methods. The most accurate methods tend to be the most expensive and may not be accessible to everyone. This does not mean the less expensive, portable devices are always inaccurate or misleading. In fact, no matter which method you choose, you'll gain valuable insights into your health and fitness.

• DEXA

  —DEXA stands for Dual Energy X-Ray Absorptiometry. This is widely considered the gold standard for measuring body composition. It works by using two low-energy levels of X-rays that differentiate between total body adipose (aka fat cells) and soft tissue, as well as bone mineral content and bone mineral density. DEXA is considered highly accurate, fast (a whole-body scan takes 10-20 minutes) and user-friendly.

• Hydrostatic weighing

  —This method measures water displacement when you are submerged in a tank of water. Hydrostatic weighing relies on the fact that lean tissue sinks in water, while fat tends to float. Therefore, the more lean mass you have, the more you will weigh underwater. This method is considered highly accurate.

• Bod Pod

  —The Bod Pod is a computerized, egg-shaped unit that uses air displacement for body composition assessment. You will need to sit inside the pod, fully enclosed for several minutes. Clothing, hair, jewelry and glasses can all interfere with the results. So, you are encouraged to wear a swimsuit or form-fitting outfit and swim cap. Like the previous two methods, this method is highly accurate and very fast (the assessment only takes about five to 10 minutes).

Is it expensive to check your body composition?

When it comes to these first three methods, it's important to bear in mind that they are expensive. And there are also individual considerations for each method. For example, DEXA cannot be used accurately for certain individuals, including women who are pregnant. Meanwhile, multiple submersions are required for hydrostatic weighing, making it time consuming, and Bod Pods can be uncomfortable for those who are sensitive to tight spaces.

Some more accessible and inexpensive methods include:

Bioelectrical impedance (BIA)

Bioelectrical impedance uses a low-energy electrical current to scan total body water, lean mass and fat mass. Muscle contains more water than fat mass, which means it will better conduct energy. The energy current will move through muscle mass faster, with little to no resistance. Rest assured, the electrical current is painless.

Devices that utilize BIA technology are safe, portable, fast, easy-to-use and readily accessible for at-home use. Before using bioelectrical impedance, remember that this method is sensitive to hydration status and may be less accurate than others.

Skinfold assessment

A skinfold assessment uses specialized calipers to measure subcutaneous fat thickness at various points along the body. Typically, skinfold measurements are taken at the stomach, side of the torso, chest, upper thighs, under the shoulder blades, hip and triceps. BIA devices and skinfold calipers are both commonly seen in big-box fitness centers.

While this method is easy-to-use and requires minimal training, caliper measurements are considered the least accurate as measurements can vary wildly between users.

Should I track my body composition? Is it worth it?

Yes, body composition is absolutely worth tracking! Healthcare providers often put emphasis on BMI, because the medical field uses BMI to classify a whether a person is at a "normal" weight. However, BMI does not provide a complete picture of health. Body mass index is simply a ratio of your height to weight. Your BMI does not distinguish between fat mass and lean muscle mass, which are better markers for health.

In fact, BMI results can be deceiving. A person with high muscle mass percentage could be misclassified because their body weight is high in relation to their height. Therefore, it's important to dive deeper than BMI by getting a complete body composition assessment. If it's easily accessible, get a body composition scan every four to five weeks. At minimum, retest every six months.

Why did I gain weight, but my body fat percentage stayed about the same?

Measuring body composition requires the use of specialized technologies, not all of which are equally accurate. That said, there are a few explanations for seeing body weight and BMI increase while body fat percentage stays stagnant.

There was a technical error

Plain and simple: the technology you're using to measure body composition may have malfunctioned. Some of these devices require regular recalibration. So, depending on your method of measurement, a technical flaw could be to blame for confusing results.

You drank a lot of water or are retaining water

Remember: bioelectrical impedance (BIA) is sensitive to hydration levels. Water is an electrical conductor, which means the electrical current will run faster through an overhydrated body. This results in a falsely lower body fat composition reading.

If you drank a lot of water before a body composition assessment, you may see a higher body weight and higher BMI but a stagnant—or even lower—body fat percentage. This can also happen when you're retaining water from eating a high-carbohydrate diet, feeling bloated or experiencing fluid changes during your menstrual cycle.

Pro tip: Be mindful of your water intake! Dehydration will lead to falsely high body fat readings.

You gained muscle mass

Some people gain weight but lose inches and don't see significant changes in body fat. This could very well be because you gained muscle mass. A pound of fat weighs the same as a pound of muscle. However, fat takes up more volume, or space, than muscle. Because muscle is denser and takes up less space, you will appear thinner (decreased inches). Unfortunately, your weight may not change—or may even go up—with this extra muscle mass.

How to keep a healthy body composition

When you make smart lifestyle choices, you can easily achieve (and maintain!) a healthy body composition.

1. Eat a healthy diet

Whether your goal is weight loss or to gain muscle, you must dial in your nutrition. With the right mix of macronutrients and micronutrients you can promote lean muscle mass while reducing excess body fat. So, what is the "right mix"?

Start with macronutrients, which include protein, carbohydrates and fats. Protein should be prioritized for building and maintaining lean muscle mass. Include protein with every meal and snack. About 25-35% of your daily calories should come from protein sources, such as chicken, turkey, fish, egg whites, low-fat dairy, beans, lentils, nuts and seeds.

Carbohydrates are your body's main source of energy. Therefore, 40-60% of your daily calories should come from carbohydrates. Ideally, at least half of your choices are complex carbohydrates, which keep you fuller for longer.

Finally, fats are important for a wide range of essential body functions, but you don't need too much to get the job done. Depending on other health factors, 15-30% of your daily calories should come from fat. The best types are unsaturated fats, which come from olive oil, avocado, fatty fish, nuts and seeds.

And don't forget about micronutrients! The best nutrition comes from food sources first. Fruits, vegetables, legumes and whole grains are rich in essential vitamins and minerals. Make sure you're getting at least three cups of non-starchy vegetables and one to two servings of fruit each day.

Finally, be mindful of your fiber intake. Fiber helps slow down digestion, which maintains already-healthy blood sugar, promotes satiety and supports regular bowel movements and a healthy gut microbiome, important for digestion. These are all important factors in weight management and maintaining a healthy body composition. Unfortunately, most adults do not get enough fiber. For optimal health, you need 14 grams of fiber for every 1,000 calories you consume.

2. Engage in regular exercise

It's recommended that adults get at least 150 minutes of moderate-intensity exercise per week, plus at least two 20-minute strength-training sessions. Moderate-intensity exercise includes brisk walking, cycling at 10 miles per hour, dancing, swimming laps, gardening, playing basketball and other similar activities.

You can also monitor your heart rate to make sure you're hitting the right intensity. Moderate-intensity exercise should increase your heart rate 50-70% higher than your resting heart rate. Cardiovascular exercise helps burn calories, while strength training builds lean muscle mass. Therefore, you need both forms of physical activity to maintain a healthy body composition.

3. Get enough sleep

There are many benefits of sleep. For instance, getting adequate shut-eye helps support mood, memory and even your ability to multitask. A good night's sleep also allows your cells to repair, which is especially important after exercising. When you're well-rested, you can give your all during your workouts. Over time, better workouts lead to better and faster results.

Not to mention, lack of sleep has been associated with increased levels of ghrelin, the hunger hormone. In other words, a sleep-deprived night sets you up for an exceptionally hungry day. If you want to get the most out of your workouts and avoid overeating, make sure you get the recommended seven to nine hours of sleep each night.

4. Hydrate, hydrate, hydrate

Water makes up a significant portion of your total body composition—and rightfully so. You need water for almost all bodily functions. In terms of body composition, staying hydrated promotes feelings of fullness. This could lead to reduced calorie intake and ultimately achieving your weight loss goals.

References

• Ekingen T, et al. "Associations between hydration status, body composition, sociodemographic and lifestyle factors in the general population: a cross-sectional study." BMC Public Health. May 2022. https://pubmed.ncbi.nlm.nih.gov/35513819/
• Taheri S, et al. "Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index." PLoS Med. December 2004. https://pmc.ncbi.nlm.nih.gov/articles/PMC535701/
• Toomey CM, et al. "A Review of Body Composition Measurement in the Assessment of Health." Topics in Clinical Nutrition. March 2015. https://journals.lww.com/topicsinclinicalnutrition/abstract/2015/01000/a_review_of_body_composition_measurement_in_the.3.aspx
• "Target Heart Rates Chart." American Heart Association. https://www.heart.org/en/healthy-living/fitness/fitness-basics/target-heart-rates
• "What Counts as Physical Activity for Adults." Centers for Disease Control and Prevention. https://www.cdc.gov/physical-activity-basics/adding-adults/what-counts.html